Room temperature operation of InxGa1-xSb/InAs type-II quantum well infraredphotodetectors grown by MOCVDD. H. Wu, Y. Y. Zhang, and M. Razeghi

Citation: Appl. Phys. Lett. 112, 111103 (2018); doi: 10.1063/1.5021646View online: https://doi.org/10.1063/1.5021646View Table of Contents: http://aip.scitation.org/toc/apl/112/11Published by the American Institute of Physics

Room temperature operation of InxGa12xSb/InAs type-II quantum wellinfrared photodetectors grown by MOCVD

D. H. Wu, Y. Y. Zhang, and M. Razeghia)

Center for Quantum Devices, Department of Electrical Engineering and Computer Science,Northwestern University, Evanston, Illinois 60208, USA

(Received 6 January 2018; accepted 3 March 2018; published online 14 March 2018)

We demonstrate room temperature operation of In0.5Ga0.5Sb/InAs type-II quantum well photodetec-

tors on an InAs substrate grown by metal-organic chemical vapor deposition. At 300 K, the detector

exhibits a dark current density of 0.12 A/cm2 and a peak responsivity of 0.72 A/W corresponding to

a quantum efficiency of 23.3%, with the calculated specific detectivity of 2.4� 109 cm Hz1/2/W at

3.81 lm. Published by AIP Publishing. https://doi.org/10.1063/1.5021646

The mid-wavelength infrared (MWIR) window between

3 and 5 lm has wide applications from military to civilian,1–3

such as aerial and satellite reconnaissance, target tracking

using heat signals, navigation and object identification, vas-

cular and cancer detection, and industrial process monitor-

ing. For all these applications, achieving higher operating

temperatures of a single element detector or imager is highly

desirable because it will reduce the cost and complexity.

During the past few years, tremendous effort has been spent

on developing high operating temperature infrared photode-

tectors, most notably mercury-cadmium-telluride (MCT),4

InAs/GaSb type-II superlattices (T2SLs),5,6 and inter sub-

band based quantum dot infrared photodetectors (QDIPs).7

Although the state-of-the-art MWIR photodetectors based on

a bulk MCT material have already demonstrated near-room

temperature operation, the MCT material is still plagued by

nonuniform growth and expensive CdZnTe substrates. InAs/

GaSb-based T2SLs are viable candidates to compete with

the state-of-the-art MCT material system due to their unique

material characteristics, such as large effective mass and

ability to suppress Auger recombination. High-performance

MWIR photodetectors based on InAs/GaSb T2SLs have

been extensively demonstrated.8,9 However, most of the

T2SL structures and devices have been grown by molecular

beam epitaxy (MBE), which is expensive for mass produc-

tion application due to the large costs of upkeep and very

slow output rates. QDIPs utilize the intraband transitions for

infrared detection. The quasi-three-dimensional confinement

of the quantum dot (QD) provides important advantages

for infrared photodetection such as inherent sensitivity to

normal-incidence light, low dark current, and high operating

temperature. Nevertheless, recently reported QDIP perform-

ances are still far below the theoretical expectation.10,11

Recently, the Sb-based InSb/InAs type-II material system

has attracted much interest in the MWIR band due to its

unique type-II band alignment which can provide low-

energy optical transitions in the III/V material system.12,13

Although the InSb/InAs material system has a large mis-

match (7%), similar to the well-known InAs/GaAs material

system, Stranski–Krastanov (S-K) mode growth of InSb on

InAs does not result in high quality QD due to the Sb

segregation and surfactant effects.14 Initial works on the

growth of a thin InSb layer on InAs have demonstrated

promising results in light-emitting diodes15 and lasers.16

More recently, efforts have been made to tune the strain and

extend the emission wavelength by adding Gallium (Ga) to

form an InxGa1�xSb thin layer on the InAs matrix for photo-

detector applications.17,18 However, the photodetectors based

on this material system are facing more challenges and there-

fore still have limited performance in recent reports.17

In this work, we report the growth of a high quality mate-

rial and demonstration of high performance MWIR photode-

tectors based on InxGa1�xSb/InAs type-II quantum well

(QW) structures on an InAs substrate grown by metal-organic

chemical vapor deposition (MOCVD), which could enable

lower-cost and versatile production. Figure 1(a) shows a

schematic of the device structure. For theoretical guidance,

we applied the k�p model to calculate the bandstructure of the

proposed type-II QW structure. Shown in Fig. 1(b) is the cal-

culated type-II energy band diagram for the In0.5Ga0.5Sb/

InAs QW associated with strain. In such In0.5Ga0.5Sb/InAs

type-II QW structures, holes are confined in the In0.5Ga0.5Sb

well while electrons are not confined in the InAs barrier. The

valence bands in the strained In0.5Ga0.5Sb well are split into

heavy- and light-hole bands. The detection mechanism is

based on spatially indirect band to band transitions between

the bound hole-states in the In0.5Ga0.5Sb well and the contin-

uous electron-states in the InAs barrier.

There are several advantages in this type of type-II QW

photodetector. The first advantage resides in the suppression

of the Auger recombination. Usually, Auger recombination

is the dominant recombination mechanism at high tempera-

tures and is one of the obstacles in realizing infrared detec-

tors operating at high temperatures. The material systems

with type-II band alignment such as InxGa1-xSb/InAs super-

lattices, quantum wells, and quantum dots were recognized

as promising materials for MWIR lasers and photodetectors

due to the predicted reduced Auger recombination rates.19–22

The proposed In0.5Ga0.5Sb/InAs type-II QW structures will

apparently have a lower Auger recombination rate in com-

parison with type-I heterostructures and bulk materials due

to their type-II band alignment. Another promising advan-

tage is the long carrier lifetime. The lifetime of minority car-

riers is an important parameter which defines both the darka)Electronic mail: razeghi@eecs.northwestern.edu

0003-6951/2018/112(11)/111103/4/$30.00 Published by AIP Publishing.112, 111103-1

APPLIED PHYSICS LETTERS 112, 111103 (2018)

current and quantum efficiency (QE) of photodetectors. In

InxGa1�xSb/InAs type-II QW structures, electrons in the

InAs barrier and holes in the InxGa1�xSb well are separated

compared to those in conventional type-I structures. The spa-

tially indirect nature of the transitions is beneficial for a long

carrier lifetime.23,24

The device was grown on an n-type InAs substrate in an

EMCORE MOCVD reactor at a pressure of 60 Torr.

Trimethylindium (TMIn) and triethylgallium (TEGa) were

used as group III precursors. Trimethylantimony (TMSb) and

AsH3 were used as group V precursors. Diethylzinc (DEZn)

and SiH4 were used for p-type and n-type dopants, respec-

tively. The growth of the device structure started with a

300 nm n-type InAs buffer layer to smooth out the surface.

Then, a 0.5 lm n-type (n� 1018 cm�3) InAs bottom contact

was grown, which was followed by the 100 period undoped

In0.5Ga0.5Sb/InAs type II QW active region and a 0.5 lm thick

p-type (p� 1018 cm�3) InAs top contact. The In0.5Ga0.5Sb

well layer was grown with a nominal thickness of 4 ML sepa-

rated by a 20 nm InAs barrier. The whole structure was grown

at 430 �C. After the growth, the sample was structurally char-

acterized using high resolution X-ray diffraction (HR-XRD),

atomic force microscopy (AFM), and transmission electron

microscopy (TEM).

Figure 2 shows the X-ray diffraction scan curve (a),

AFM image (b), and TEM (c) characterization results of the

grown device structure. In the XRD curve, satellite peaks up

to 7th-order are clearly observed, which indicates an excel-

lent structural quality. The full width at half maximum

(FWHM) values at 0th and 1st satellite peaks are only 30 and

54 arc sec, respectively. The overall period is 20.1 nm, which

is in good agreement with the designed value. The lattice

mismatch between the InAs substrate and the active region is

1556 ppm. The sample exhibits a good surface morphology

with clear atomic steps. The root mean square (RMS) rough-

ness is only 1.1 A for the 5� 5 lm2 scan area. Figure 2(c) is

the cross-sectional dark-field TEM image of a representative

portion of the active region in our device using the (002)

reflection. The In0.5Ga0.5Sb layer looks like an ultrathin pla-

nar layer and shows no evidence of defect. The XRD, AFM,

and TEM results suggest excellent material quality for our

detector device.

The material was subsequently processed into mesa

isolated single element diodes with device sizes ranging

from 100� 100 to 400� 400 lm2 using a standard photo-

lithographic processing technique followed by mesa defini-

tion using BCl3:Arþ dry etching and citric acid treatment to

remove dry etch residues. Top and bottom metal contacts

were formed using electron beam deposited Ti/Au. Then, the

devices were wire-bonded onto a 68 pin leadless chip carrier

(LCCC) and loaded into a cryostat for both optical and elec-

trical characterizations in the temperature range from 150 to

300 K.

The device was optically characterized under front-side

illumination without any anti-reflection (AR) coating applied

to the devices. The spectral response was measured using a

Bruker IFS 66v/S Fourier transform infrared spectrometer

(FTIR), and the responsivity (Ri) and quantum efficiency

(QE) were measured with a calibrated blackbody source at

1000 �C. The device shows zero bias dependent performance.

The Ri and QE spectra of the device measured at zero bias

are shown in Figs. 3(a) and 3(b) for temperatures of 150 and

300 K. As we can see, there are two distinctly different photo-

responses in the spectra. The short wavelength photoresponse

FIG. 1. (a) Schematic layer design of

the In0.5Ga0.5Sb/InAs type-II QW pho-

todiode; (b) schematic energy band

diagram of In0.5Ga0.5Sb QW with the

InAs barrier.

FIG. 2. (a) High-resolution X-ray diffraction of the grown device structure;

(b) the atomic force microscopy image of a 5� 5 lm2 surface area of the

grown device structure with a RMS roughness value of 1.1 A; (c) DF-TEM

image of In0.5Ga0.5Sb/InAs QW in the grown device structure.

111103-2 Wu, Zhang, and Razeghi Appl. Phys. Lett. 112, 111103 (2018)

with an approximate 3.5 lm cut-off originates from the direct

bandgap transitions in the InAs barrier material,24 while the

photoresponse with an approximate 5.0 lm cut-off is due to

the type-II transitions from the bound hole states in the

In0.5Ga0.5Sb well to the conduction band states in the InAs

barrier.

The photodetectors exhibit a 100% cut-off wavelength

of �4.50 lm at 150 K; the responsivity peaks at 0.65 A/W,

corresponding to a QE of 23.2% at 3.48 lm for the 100

period In0.5Ga0.5Sb/InAs type-II QW active region. At

300 K, the detector shows a 100% cut-off wavelength of

�5.22 lm; the peak responsivity is 0.72 A/W at 3.81 lm,

corresponding to a QE of 23.3%. The temperature depen-

dent optical performance of the device was also character-

ized. The inset of Fig. 3(b) shows the peak QE value of the

photoresponse from type-II QW at the peak responsivity

wavelength from 150 to 300 K. The type-II QW related pho-

toresponse QE increases gradually with temperature, and

the highest QE of 33.4% is obtained around 220 K. For tem-

perature above 220 K, the QE starts to decrease. In a p-i-n

photodiode, QE is determined primarily by the absorption

coefficient of the material and diffusion length of minority

carriers. For the In0.5Ga0.5Sb/InAs type-II QW device struc-

ture, the absorption coefficient increases with increasing

temperature. This leads to an increase in QE when tempera-

ture increases from 150 to 220 K. With the temperature

increased to 200 K above, the minority carrier lifetime starts

to decrease due to the stronger Auger recombination. This

will result in the decrease in the minority carrier diffusion

length, which is the main reason for decreased QE at high

temperature.

For the temperature dependent electrical performance

measurement, a cold shield was placed in front of the photo-

detectors. Figure 4(a) exhibits the dark current density as a

function of the applied bias voltage at different temperatures

from 150 to 300 K. At 150 K and �20 mV bias, the photode-

tector shows a dark current density and differential resistan-

ce� area (R�A) product of 1.5� 10�7 A/cm2 and 200 876

X cm2, respectively. At 300 K, the dark current density and

R�A product at �20 mV bias are 0.12 A/cm2 and 0.2 X cm2,

respectively. An Arrhenius plot of the dark current density

versus the inverse temperature (1/T) from 150 to 300 K under

�20 mV applied bias voltage is shown in Fig. 4(b). An acti-

vation energy of about 340 meV was obtained, which corre-

sponded roughly to the InAs bandgap (350 meV). This

indicates that the dark current is diffusion current limited

assisted via the InAs barrier.

After the electrical and optical characterization of the

device, the specific detectivity (D*) was calculated using the

following equation:

D� ¼ Ri 2qJ þ 4kbT

R� A

� ��1=2

; (1)

where q is the electronic charge, kb is Boltzmann’s constant,

and T is the temperature of the device. Shown in Fig. 5 are

the calculated specific detectivity (D*) of the device based

on the measured responsivity, the dark current density, and

the R�A product at different temperatures under �20 mV

applied bias. At 150 K, a peak detectivity D* of 6.2� 1011

cm Hz1/2/W was calculated at peak responsivity (k¼ 3.48 lm)

and the device exhibited a peak detectivity D* of 2.4� 109

cm Hz1/2/W at peak responsivity (k¼ 3.81 lm) at 300 K. The

In0.5Ga0.5Sb/InAs type-II QW MWIR detectors have demon-

strated excellent electrical and optical performance at room

temperature.

In conclusion, we reported an In0.5Ga0.5Sb/InAs type-II

QW MWIR photodetector grown on an InAs substrate by

MOCVD. The detector shows a peak responsivity of 0.65 A/

W at the wavelength of 3.48 lm and a dark current density

of 1.5� 10�7 A/cm2, with the calculated specific detectivity

of 6.2� 1011 cm Hz1/2/W at 150 K. At 300 K, the peak

responsivity is 0.72 W/A at 3.81 lm and the dark current

density is 0.12 A/cm2, with the calculated specific detectiv-

ity of 2.4� 109 cm Hz1/2/W. Our results suggest great poten-

tial for the realization of high performance MWIR

FIG. 3. (a) Responsivity spectra and

(b) quantum efficiency spectra of the

photodetector under zero bias at 150

and 300 K. Inset of (b): The peak quan-

tum efficiency value of the photores-

ponse from InGaSb/InAs Type-II QW

from 150 to 300 K.

FIG. 4. (a) Dark current density as a function of applied bias voltage mea-

sured at different temperatures from 150 to 300 K; (b) Arrhenius plot of the

dark current density of the detector under �20 mV applied bias.

111103-3 Wu, Zhang, and Razeghi Appl. Phys. Lett. 112, 111103 (2018)

photodetectors based on the In0.5Ga0.5Sb/InAs type-II QW

structure on the InAs substrate.

The authors would like to acknowledge Dr. Yaobin Xu

and Professor Vinayak P. Dravid from the Department of

Materials Science and Engineering at Northwestern

University for the TEM measurement.

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FIG. 5. Specific detectivity (D*) spectra of the detector under �20 mV

applied bias at different temperatures from 150 to 300 K.

111103-4 Wu, Zhang, and Razeghi Appl. Phys. Lett. 112, 111103 (2018)